Pneumatic tyre

Abstract: A pneumatic tyre for a vehicle comprising a carcass ply (4) extending between two bead regions and passing through a tread region (1) which extends between two tread contact edges, a tread region (1) reinforcing breaker (2) radially outwards of the carcass (4) in the tread region (1) and a ground contacting tread surface (10) curved in the axial direction characterised in that the curvature of the outer tread surface (10) when the tyre is mounted on a wheel rim (11) and normally inflated has a continuously decreasing radius from a point P which is at a distance SP in the axial direction from the tyre circumferential centreline (C) equal to 20% of the distance from the tread centre to the adjacent tread contact edge.

Description: The present invention relates to a pneumatic tyre for vehicles having a radial tyre and a tread reinforcing breaker and is particularly applicable to car tyres but not limited thereto. Current vehicles are being made progressively lighter and great demands are made by vehicle makers to lighten tyres and improve their other properties including handling, wet grip, etc. The present radial tyre development has been concentrated on making the tyre footprint consistent under all conditions but this has resulted in tyres with wide flat tread regions for high cornering power in the dry but which are inefficient in relation to wet grip. It is an object of the present invention to provide a pneumatic tyre which overcome these problems and provides a tyre of reduced weight. According to the present invention a pneumatic radial tyre for a vehicle comprises a carcass ply extending between two bead regions and passing through a tread region which extends between two tread contact edges, a tread region reinforcing breaker radially outwards of the carcass in the tread region and a ground contacting tread surface curved in the axial direction characterised in the curvature of the outer tread surface when the tyre is mounted in a wheel rim and normally inflated has a continuously decreasing radius from a point P which is at a distance SP in the axial direction from the tyre circumferential centreline equal to 20% of the distance from the tread centre to the adjacent tread contact edge. By tread contact edge is meant the edge point axially of the tyre at which the tread surface is contacted by the road surface in use of the tyre. The tread contact surface may have a radius of curvature which decreases at a constant rate. Alternatively the curvature may be defined by the locus of the point PT with polar co-ordinates R, having an origin on the circumferential centreline C at a distance below the point of intersection of the circumferential centreline C with the tread surface wherein R = (92.46304 + 50.02951 x  - 109.1216 x  2 + 43.74487 x  3 + 7.385639 x  4 - 4.776894 x  5 ) x (SW/194) as  decreases in the range from π/2 radians to zero and wherein SW is the value in millimetres of the maximum section width of the tyre. Preferably the polar co-ordinates R, have an origin on the circumferential centreline C at a distance of (70.63044 x (SW/194))mm below the point of intersection of the circumferential centreline C with the tread surface. In a broader aspect of the shape the invention includes a tread surface curve lying within two curves generated with the above ±4% in relation to the radius. More preferably the curve is within ±2% of the radius. Another aspect of the invention provides a pneumatic tyre for a vehicle comprising a carcass ply extending between the bead regions and passing through a tread region which extends between two tread contact edges, a tread region reinforcing breaker radially outwards of the carcass in the tread region and a ground contacting tread surface curved in the axial direction characterised in that the outer surface of the breaker has a curved shape in the tyre axial direction and has a decreasing radius from a point B to the edge of the breaker wherein point B is spaced apart from the tyre circumferential centreline by a distance SP equal to 20% of the distance from the tread centre to the tread contact edge. Once again the particular profile may be defined by an equation relating the polar co-ordinates and in broader aspects ranges of ±4% and more preferably ±2% of the radius. Whilst the invention relates primarily to a tyre having a tread region and/or a breaker construction having the declared curvature the said curvature in a preferred arrangement continues along the sidewall regions of the tyre substantially from bead region to bead region. Yet another aspect of the invention provides that the thickness of the tread rubber decreases from a point P on the tread surface to the tread edge wherein the point P is set at a distance SP from the circumferential centreline C being equal to 20% of the distance between the circumferential centreline and the tread edge. Further aspects of the present invention will become apparent from the following description by way of example only, of one embodiment of the invention in conjunction with the attached drawings in which:- Figure 1 is a cross-section of a tyre with the left-hand part of the drawing showing the construction features and the right-hand part of the drawing the geometrical features of the surface of the tyre; and Figure 2 is a cross-section of a tyre with the left-hand part of the drawing showing the construction features and the right-hand part of the drawing the geometrical features of the surface of the outer breaker ply. The tyre of Figure 1 comprises a 195/45R16 car tyre, for a 6½" x 16" wheel rim, and has a tread region 1 reinforced by a breaker assembly 2 and a pair of sidewalls 3. A single ply of radially extending cords provides a carcass reinforcement 4 which extends from bead to bead of the tyre and is wrapped at each edge from the inside to the outside in the axial direction around a bead core 5. The edge region 6 of the ply 4 is turned up radially outwards to give a ply turnup. A rubber apex 7 is positioned on top of the bead core 5 in a conventional manner and a chafer 8 is wrapped around the bead 5 inwards of the ply 4 to provide a cushion for the ply around the bead core 5 and apex 7. A clinch strip 9 is also positioned around the bead. The components in the bead provide a substantially conventional bead region. The tyre carcass has a substantially curved cross-sectional shape to match the breaker package which in turn matches a substantially curved tyre tread surface of the tread region 1. The curvature of the tread region is one of the important features of this invention and reference to the right-hand side of the Figure shows that the tread region has a gradually decreasing radius of curvature moving outwards from the centreline of the tyre. More specifically the tread region has a radius of curvature which continually decreases to either side of the centreline outwards of a point P on the tread surface which is a distance equal to 20% of the distance from the centreline C to the tread contact edge point. The embodiment has a tread surface shape with the progressively reducing radius of curvature RC defined by an equation which gives the locus of a point PT using polar coordinates R and  having an origin O on the centreline of the tyre cross-section C substantially below the centre of the tyre tread surface and at a similar height to the top of the flange for the wheel rim 11 on which the tyre is mounted. It should be understood however that some variation on the curve is possible. Thus the shape for a practicable tyre may be within ±4% of the radius according to the detailed formula which follows and relates specifically to this embodiment. More preferably the shape needs to be within ±2%. The flange height G provides a contact point BP where the tyre sidewall touches the flange. The flange height G for the tyre and wheel of the assembly of a body 1 is 17.3mm. The centre O was set at a height from the bead seat diameter D of 20.4mm. The height of the point O is not critical but needs to be set so that the locus can be calculated. For the embodiment the equation to give the locus which is the tyre tread surface shape is R = (92.46304 + 50.02951 x  - 109.1216 x  2 + 43.74487 x  3 + 7.385639 x  4 - 4.776894 x  5 ) x (SW/194) where  decreases in the range from π/2 radians to zero, SW is the value of millimetres in the maximum section width of the tyre and the origin O is a distance of 70.63044 x (SW/194) millimetres below the point of intersection in the circumferential centreline C with the tread surface. It must be noted that this applied from the point P along the surface of the full width of the tyre tread 1 to give the tyre tread surface 10 having the shape defined. The shape may continue to the edge of the tread rubber. However, in the embodiment the shape continues around the sidewall of the tyre substantially to the point BP where the tyre sidewall contacts the flange of its wheel rim. Thus the sidewalls 3 have the outer profile shape defined by the same equation as the tread. To clarify the width of the tread (SW) reference should be made to the drawing where the tread region 1 can be seen to extend to substantially the mid-sidewall point which is the widest part of the tyre. Thus in this embodiment the tread width SW is the section width of the tyre because the tread region extends in this way. This is not an essential feature of the invention for the tread width to equal the section width but is preferred. The above relates to the shape of the tread surface. Figure 2 utilises the same numerals to show a construction in which the curvature of the top breaker ply 2B is defined by a similar equation. In this case the curvature of the outer surface of the widest breaker ply is defined by the locus of a point PT with polar co-ordinates R B , having an origin on the circumferential centreline C at a distance below the point of intersection of the circumferential centreline C with the tread surface wherein R B = (89.02495 + 58.35249 x  - 194.2836 x  2 + 168.7756 x  3 - 62.10578 x  4 + 8.747225 x  5 ) x (SW/194) as  decreases in the range from π/2 radians to zero and wherein SW is the value in millimetres of the maximum section width of the tyre. Once again whilst this is the preferred shape the invention also includes a shape generated by polar co-ordinates where the radius is within ±4% or more preferably ±2% of the above curve radius. The tread surface has the profile set out in its general shape. This does not exclude the provision of pattern grooves in the surface from normal sipes which are very narrow cuts to quite wide water-retaining grooves in either or both of the circumferential or axial directions. The tread pattern may also include a very wide centre groove of the type currently fashionable. The tread region 1 has a rubber compound thickness measured perpendicularly to the surface of the tread which reduces continuously from the centre (C) of the tread to each shoulder. This type of tread thickness is unconventional and typically radial tyres have an increasing or constant tread thickness towards the shoulder even when the shoulders themselves are rounded to give even wear appearance on the tyre in service. The tread reinforcing breaker of the present tyre comprises three plies, 2A, 2B and 2C. Beginning with the radially innermost ply 2A the construction comprises an inner breaker ply 2A having a width B1 of 180mm measured in the axial direction of the tyre. Thus the outer breaker ply in common with the others lies substantially parallel to the tread surface profile 10 which means that the ply itself is cut substantially wider than 180mm. The next ply 2B has a width B2 which in the finished tyre is 180mm measured in the axial direction and is loaded with cords at an opposite acute angle to the first ply. Finally a third breaker ply 2C is positioned centrally of the tyre and has a finished axial width of 150mm so that its edges do not reach to the edges of the first and second plies 2A and 2B. The three breaker plies 2A, 2B and 2C all comprise aramid cords material (Kevlar Registered Trade Mark) each cut at 25° prior to manufacture. The cord plies are topped with topping compound. Thus in the embodiment an important feature of the invention is that the profile of the breaker 2 is the same as that of the tread surface 10 so that the package of tread surface, tread thickness and breaker each follow the declared shape. Finally under the breaker the radial reinforcement carcass 4 comprises nylon reinforcing cords laid substantially at 90° and lying adjacent to the inner breaker ply 2A. As has been explained the resultant tyre has a reinforced tread region of substantial curvature when considered in transverse cross-section. For example in the embodiment shown in Figure 2 the tread curvature is such that the ratio TH/SW of the radial distance between the tread edge and the tread centre to the maximum tyre section width has a value of 0.27. This is very different to normal car and truck tyres where the intention of the designer has hitherto been to make the breaker as flat a possible to maintain a flat contact between the tread region and the road. In the present tyre the combination of tread and breaker shape together with the tread thickness being substantially constant or in fact diminished across the full width of the tyre to the shoulders provides a tyre in which when the vehicle is running straight the contact patch is substantially less that the width of the tyre. Thus the tyre has a contact patch which can move to either side of the centreline and/or increase in transverse width with lateral/radial changes of load on the wheel concerned. Initial testing of this new tyre has shown that cornering power is substantially improved compared with a traditional radial tyre having a flat tread and breaker package. This is believed to be due to the contact patch moving across the tyre structure and the tyre structure adapting itself to the forces applied thereto. The tyre also has a very good runflat performance. Whilst the embodiment described comprises aramid (Kevlar) cords for the breaker and nylon carcass cords the invention is not limited to such materials. Any of the normal high tensile materials can be used for the breaker package as can two, three, four or more plies. The carcass may also comprise other materials including rayon and may be of single or dual ply construction. The tyre described is a 45 aspect ratio tyre and therefore comprises short sidewalls. The invention however is not limited thereto as it resides primarily in the shape of the tread surface and the breaker package thereunder. Similarly the bead construction may be varied including making the apex strip of different sizes and hardnesses and any of the known bead retention systems may be utilised.

Claims: A pneumatic tyre for a vehicle comprising a carcass ply (4) extending between two bead regions and passing through a tread region (1) which extends between two tread contact edges, a tread region reinforcing breaker (2) radially outwards of the carcass (4) in the tread region (1) and a ground contacting tread surface (10) curved in the axial direction characterised in that the outer tread surface (10) has a curvature when the tyre is mounted on a wheel rim (11) and normally inflated which has a continuously decreasing radius (R) from a point P which is at a distance SP in the axial direction from the tyre circumferential centreline (C) equal to 20% of the distance from the tread centre to the adjacent tread contact edge. A pneumatic tyre according to claim 1, characterised in that the continuously decreasing radius (R') continues to the edges of a tread rubber component at each side of the tyre. A pneumatic tyre according to claim 1, characterised in that the continuously decreasing radius (R') continues to the point of maximum axial width of the tyre at each side of the tyre. A tyre according to any of claims 1 to 3, characterised in that the tread surface (10) has a radius of curvature which decreases at a constant rate. A tyre according to any of claims 1 to 4, characterised in that the tread surface (10) is a curve lying within the two curves defined by the locus of a point with polar co-ordinates R and  where R = R ±4% R wherein R = (92.46304 + 50.02951 x  - 109.1216 x  2 + 43.74487 x  3 + 7.385639 x  4 - 4.776894 x  5 ) x (SW/194) as  decreases in the range from π/2 radians to zero and wherein SW is the value in millimetres of the maximum section width of the tyre. A tyre according to any of claims 1 to 4, characterised in that the tread surface (10) is a curve lying within the two curves defined by the locus of a point with polar co-ordinates R and  where R = R ±2% R wherein R = (92.46304 + 50.02951 x  - 109.1216 x  2 + 43.74487 x  3 + 7.385639 x  4 - 4.776894 x  5 ) x (SW/194) as  decreases in the range from π/2 radians to zero and wherein SW is the value in millimetres of the maximum section width of the tyre. A tyre according to any of claims 1 to 4, characterised in that the tread surface (10) has a curvature defined by the locus of a point PT with polar co-ordinates R, having an origin on the circumferential centreline C at a distance below the point of intersection of the circumferential centreline C with the tread surface wherein R = (92.46304 + 50.02951 x  - 109.1216 x  2 + 43.74487 x  3 + 7.385639 x  4 - 4.776894 x  5 ) x (SW/194) as  decreases in the range from π/2 radians to zero and wherein SW is the value in millimetres of the maximum section width of the tyre. A tyre according to claim 5, 6, or 7, characterised in that the polar co-ordinates R', have an origin on the circumferential centreline C at a distance of (70.63044 x (SW/194))mm below the point of intersection of the circumferential centreline (C) with the tread surface. A tyre according to any of claims 1 to 8, characterised in that the curvature of the outer surface (10) of the tyre in the axial direction from the point P on the tread of the tread edge is continued substantially to a point BP on the tyre surface in the bead region (5) which is the point at which the tyre surface has its profile adapted to fit the wheel rim (11). A pneumatic tyre for a vehicle comprising a carcass ply (4) extending between the bead regions (5) and passing through a tread region (1) which extends between two tread contact edges, a tread region reinforcing breaker (2) radially outwards of the carcass (4) in the tread region and a ground contacting tread surface (10) curved in the axial direction characterised in that the outer surface of the breaker (2) has a curved shape in the tyre axial direction and has a decreasing radius (R B ) from a point B to the edge of the breaker wherein point B is spaced apart from the tyre circumferential centreline by a distance SP equal to 20% of the distance from the tread centre (c) to the tread contact edge. A tyre according to claim 10, characterised in that the outer surface of the breaker (2) has a radius which decreases at a constant rate from the point B to the breaker edge. A tyre according to claim 10 or 11, characterised in that the decreasing radius extends from the tread centreline to the tread contact edge. A tyre according to any of claims 10 to 13, characterised in that the outer surface of the breaker (2) is a curve lying within the two curves defined by the locus of a point with polar coordinates R B and  where R B = R B ±4% R B wherein R B = (89.02495 + 58.35249 x  - 194.2836 x  2 + 168.7756 x  3 - 62.10578 x  4 + 8.747225 x  5 ) x (SW/194) as  decreases in the range from π/2 radians to zero and wherein SW is the value in millimetres of the maximum section width of the tyre. A tyre according to any of claims 10 to 13, characterised in that the outer surface of the breaker (2) is a curve lying within the two curves defined by the locus of a point with polar coordinates R B and  where R B = R B ±2% R B wherein R B = (89.02495 + 58.35249 x  - 194.2836 x  2 + 168.7756 x  3 - 62.10578 x  4 + 8.747225 x  5 ) x (SW/194) as  decreases in the range from π/2 radians to zero and wherein SW is the value in millimetres of the maximum section width of the tyre. A tyre according to any of claims 10 to 12, characterised in that the curvature of the outer surface of the widest breaker ply (2B) is defined by the locus of a point PT with polar co-ordinates R B , having an origin on the circumferential centreline C at a distance below the point of intersection of the circumferential centreline C with the tread surface wherein R B = (89.02495 + 58.35249 x  - 194.2836 x  2 + 168.7756 x  3 - 62.10578 x  4 + 8.747225 x  5 ) x (SW/194) as  decreases in the range from π/2 radians to zero and wherein SW is the value in millimetres of the maximum section width of the tyre. A tyre according to claim 13, 14 or 15, characterised in that the polar co-ordinates for the breaker have an origin on the circumferential centrelines C at a distance of (60.99363 x (SW/194))mm below the point of intersection of the circumferential centreline C with the breaker. A tyre according to any of claims 1 to 16, characterised in that the breaker (2) comprises at least two breaker plies (2A,2B) of fabric comprising parallel cords laid at an angle of 5° to 60° to the tyre circumferential direction. A tyre according to claim 17, characterised in that the cords in adjacent breaker plies (2A,2B) are crossed with respect to each other. A tyre according to claim 16 or 17, characterised in that the cords in the adjacent plies (2A,2B) are inclined at the centreline of the tyre at an angle of 25° to the tyre circumferential direction. A tyre according to any of claims 1 to 19, characterised in that the breaker fabric cords comprise aromatic polyamide. A tyre according to any of claims 1 to 20, characterised in that the radially innermost breaker ply (2B) is narrower than the adjacent breaker ply. A radial tyre according to any of claims 1 to 21, characterised in that the breaker comprises three plies (2A,2B,2C) of breaker fabric comprising cords. A radial tyre according to claim 22, characterised in that the radially outermost breaker ply (2C) is narrower than the other plies (2A,2B). A tyre according to any of claims 1 to 23, characterised in that the ratio C B /B2 between the radial distance C B from the centre (C) to the edge of the breaker and the axial distance B2 from the centre to the edge of the breaker (2) has a value between 0.3 and 0.6. A tyre according to any of claims 1 to 24, characterised in that the carcass ply (4) comprises nylon cords. A tyre according to any of claims 1 to 25, characterised in that the tyre further comprises sidewall regions (3) extending between each bead region and the tread region. A tyre according to claim 26, characterised in that the radial height of the junction of the sidewall and tread above the base of the tread bead is between 25% and 50% of the radial height of the centre of the tread outer surface above said tyre bead base. A tyre according to any of claims 1 to 27, characterised in that the ratio TH/SW between the radial distance CT from the centre to the edge of the tread and the tyre maximum width SW has a value in the range of 0.15 to 0.3. A tyre according to any of claims 1 to 28, characterised in that the thickness of the tread region (1) decreases from a point P on the tread surface to the tread edge wherein the point P is set at a distance SP from the circumferential centreline C being equal to 20% of the distance between the circumferential centreline and the tread edge. A tyre according to any of claims 1 to 29, characterised in that the carcass (4) comprises a ply of tyre cords extending in the radial direction and the ply extends substantially parallel to the outer surface of the widest breaker ply. A tyre according to any of claims 1 to 30, characterised in that the aspect ratio SH/SW of the tyre maximum section height SH to the tyre maximum section width SW is (G/SW) + 0.38015 wherein G is the height of the flange of the wheel rim on which the tyre is to be fitted.

Inventors: CLARKE DAVID HENRY, NOCK NIGEL GERARD, WILLIAMS ARTHUR ROGER, CLARKE, DAVID HENRY, NOCK, NIGEL GERARD, WILLIAMS, ARTHUR ROGER
Applicants: SUMITOMO RUBBER IND, SUMITOMO RUBBER INDUSTRIES LTD.
Application year: 1994
Main classification: B60C11
IPC codes: B60C11, B60C3, B60C9